I think the solution given by codeacrobat is the best one, but if you wanted to insist on working with the one scalar value containing multiple lines, maybe something like this:

$a = "
a 10
b a2 s2
c 30";

( $c ) = $a =~ /^b[ \t]+(.*)/m;

print "$c\n";
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That is, simply make sure you don't include "\n" among the kinds of white-space that can follow "b" in order to yield a match.

(BTW, you want everything on the line that starts with "b", and the "m" modifier on the regex does not affect the behavior of "." -- it still will not match "\n", so the question mark and dollar sign in the OP version -- (.*?)$ -- are redundant here.)